In the past, Reference Frequency Generating devices, or Frequency References, have been critical to the operation of most communication, navigation, and Electronic Warfare products. These frequency references are used to provide stable frequency reference for RF communication and provide stable time base for timing critical applications, such as GPS.
Most frequency references are designed to maintain accuracy over temperature by either controlling temperature (i.e. operate in an ovenized environment) or by compensating for temperature (using a control loop).
In many controlled crystal oscillators that utilize a control loop, a fast reaction time is needed to be able to track rapidly-changing temperature changes. To have a fast reaction time, the sample time, or the time it takes to have the next sample for a digital system, needs to be short. However, to minimize quantization error in a frequency measurement, the gate time, or the time it takes to perform the measurement, needs to be long. In a high accuracy frequency reference that is temperature compensated, there needs to be a short sample time to track errors, but a long gate time to make measurements accurate enough to correct for errors. In current state-of-the-art reference frequency generating devices that use temperature compensation (e.g.—Microcontroller Compensated Crystal Oscillators or “MCXOs”) Sample Time and Gate Time are the same value: i.e., one frequency measurement per one sample time and one frequency measurement in one gate time.
The sample time/gate time of these devices is chosen as a design tradeoff.
A longer frequency measurement could be used if more accuracy is needed, and a shorter frequency measurement might be used if a more rapid response is needed (fast temperature deviations).
It has also been known in the past to use MCXOs that employ some microprocessor predictive control, and these have enjoyed considerable success. Some prior art products (e.g. Rockwell Collins (Cedar Rapids, Iowa USA) TCCOs (277-0741-0X0, 277-0751-0X0, 277-0728-0X0; 10+total)) use a “look-ahead” function that can preempt temperature changes while keeping longer gate times for more accurate measurements. These “look-ahead” functions are limited to working with temperature deviations at a specific rate and in a specific direction (i.e., temperature going hot to cold or cold to hot).
However, in many applications, very small-sampled times are needed to provide very rapidly-changing controls.
The present invention overcomes some long-standing problems associated with rapid control of frequency references, especially when long gate times are needed to provide a high level of confidence in the validity of the measurement.